1. Field of the Invention
This invention relates to a sheet feeding device suitable for use as a feeding device in an image forming apparatus such as a copying apparatus.
2. Description of the Prior Art
In image forming apparatuses such as copying apparatuses, it has usually been the practice that paper sheets of a predetermined format each having a weight of about 60 to 90 g/m.sup.2 are used and superposed one upon another in a cassette or other suitable reservoir and the cassette or the like is received in the image forming apparatus and such superposed paper sheets are taken out one by one from the uppermost paper sheet.
FIG. 1 of the accompanying drawings shows the essential portions of a conventional paper feeding device used in an image forming apparatus. In a cassette C brought to a predetermined position in the image forming apparatus, there are received a number of sheets of copying paper P superposed on a bed plate 4 biased upwardly by a spring 3. The sheets of copying paper P are maintained in place with the opposite corners of the leading end thereof restrained by separating pawls 2 (only one of which is shown). When the cassette is within the image forming apparatus, a friction ring 1a disposed around a paper feeding roller 1 provided in the copying apparatus bears against the uppermost sheet of copying paper and, when the paper feeding roller 1 is rotated clockwise in timed relationship with the various portions of the apparatus, one sheet of copying paper somewhat flexes because the leading end edge of the copying paper is held by the separating pawls, and then that copying paper is disengaged from the separating pawls due to the rigidity of the paper itself and thereafter is moved onto a conveyance path 5 by the paper feeding roller.
Such a paper feeding system is suitable in the case of copying paper having a predetermined range of weight as previously described, but especially in a case where thick paper having a greater weight is to be fed, the cooperative operation of the separating pawls and the paper feeding roller does not suitably act and thus, the paper unavoidably fails to be well disengaged from the separating pawls and is broken or cannot be fed. Accordingly, in such a case, paper feeding has often been effected manually.
However, recently, compact image forming apparatuses producing a relatively small number of image bearing sheets and suitable for use by the users have become used habitually and along therewith, the cases where copying is effected on a considerable number of small-sized thick sheets such as post cards by the use of a copying apparatus instead of printing have increased.
As the means for handling such thick sheets, there has already been proposed the one as schematically shown, for example, in FIG. 2 of the accompanying drawings. This means is basically similar to that shown in FIG. 1 in that a number of paper sheets are received in a cassette and are successively fed by the use of a paper feeding roller, but in this means, no separating pawl is provided and a front regulation plate 6b is disposed at the leading end edge of paper sheets and an upper regulation plate 6a is disposed on the upper surface of the paper sheets and a gap 6c which will permit a paper sheet to pass therethrough is provided therebetween so that the paper sheets may be taken out one by one from the gap 6c by the paper feeding roller 1.
However, it can be readily understood that in the means of such construction, paper feeding cannot be accomplished successfully if paper sheets are curled. Also, if an attempt is made to place a superposed matter on the leading end of the paper sheets to suppress the curl thereof or an attempt is made to strengthen a spring 3 to forcibly correct the curl of the paper sheets, the resistance becomes greater and this also has unavoidably led to a disadvantage that feeding of the paper sheets cannot be accomplished smoothly.
To eliminate such a disadvantage, a construction in which superposed sheet materials may be successively taken out from the lowermost sheet material as shown, for example, in FIG. 3 of the accompanying drawings has already been proposed.
FIG. 3 shows a case where the feeding device is applied to the paper feeding device of an image forming apparatus. In a cassette C receiving therein a number of sheets of copying paper, especially, thick paper sheets, and mounted at a predetermined position within the image forming apparatus, a front regulation plate 7 is provided so as to have a gap 7a which permits a sheet of copying paper to pass therethrough, at the front lower end of the cassette as viewed in the direction of feeding of the copying paper, and a slot 8a extending in the direction of feeding of the copying paper is provided in the bottom plate 8 of the cassette.
A lever 10 is pivotably mounted on a pin 9 provided at a suitable location in the image forming apparatus, and a slot 10a is formed in one end (the lower end as viewed in the Figure) of this lever, a shaft 12 being slidably engaged with this slot 10a. Further, the shaft 12 is fitted in a slot 17a (which extends parallel to the direction of feeding of the copying paper at a suitable location in the bed portion on which the cassette rests or other immovable portion 17 of the image forming apparatus), and the shaft 12 is designed to be slidable but not rotatable in the slot 17a. FIG. 4 of the accompanying drawings shows an example of the mechanism therefor. A cut-away portion 12a is formed flatly in a portion of the shaft 12 and the edge portion of the slot 17a is adapted to be engaged with the flat cut-away portion, and the shaft 12 is movable in the slot 17a in a direction perpendicular to the plane of the drawing sheet of FIG. 4 but is not rotatable because the cut-away portion 12a is engaged with the edge portion of the slot 17a. Of course, the means for rendering the shaft 12 slidable but unrotatable is not restricted thereto.
Further, at least one paper feeding roller 13 is rotatably disposed on the shaft 12 and a one-way clutch 14 is secured thereto. This clutch may be engaged and disengaged with the paper feeding roller 13 to rotate the same roller only in the direction of arrow B in FIG. 3.
On that side of the lever 10 which is opposite to the side on which the paper feeding roller is disposed, the other end of a spring 11 having one end secured to a suitable location in the image forming apparatus is secured, whereby the lever 10 is biased in the direction indicated in FIG. 3. Further, the end portion 10b of the lever 10 bears against a movable member 15 such as the movable original supporting table of the image forming apparatus.
The paper feeding device is generally of the above-described construction. As the movable member 15 is moved from its solid-line position of FIG. 3 to its dots-and-dash line position in timed relationship with the operation of the various members in the image forming apparatus, the lever 10 pivots to its dots-and-dash line position. Along therewith, the paper feeding roller 13 at the other end of this lever is not rotated but is moved leftwardly along the slot 10a of the lever and the slot 17a provided in the immovable portion of the apparatus. At this time, the one-way clutch 14 is engaged with the paper feeding roller 13 and does not rotate the latter and thus, the lowermost sheet of copying paper in the cassette is discharged through the gap 7a due to the friction between this roller and said lowermost sheet of copying paper.
After the discharged copying paper has been reliably fed into the image forming apparatus, the lever 10 is returned to its solid-line position of FIG. 3 by the action of the spring 11, but at this time, the engagement between the paper feeding roller 13 and the one-way clutch 14 is released and therefore, the roller 13 is free to rotate and accordingly, the friction thereof with the copying paper in the cassette which bears thereagainst is small and the lever can readily return to said solid-line position without affecting the copying paper in any way.
As described previously, where thick copying paper sheets are received in the cassette, if the copying paper sheets are curled, the lowermost paper sheet in the cassette is not opposed to the discharge gap 7a of the cassette as shown in FIG. 5 of the accompanying drawings and therefore, paper feeding would not be accomplished successfully. Recently, image forming apparatuses in which a cartridge comprising a plurality of photosensitive mediums and developing devices containing developers of different colors is prepared and this cartridge is interchanged to obtain copies of plural colors have become often used, but in such a case, curling of the copying paper is liable to occur when it is heated to fix the image of a first color, and such curled portion is liable to hamper the paper feeding when automatic paper feeding is effected again by the paper feeding device for the formation of the image of a second color.
In order to eliminate such an inconvenience, it would occur to mind to place a weight on top of the copying paper sheets in the cassette and forcibly correct the produced curl temporally, but if this is done, the load applied to the paper feeding roller will become great and also, even if a weight is applied only to the forward portion of the copying paper sheets in the cassette, the burden of the conveyor roller in the conveyance path applied to a discharged sheet of copying paper will become great, and this is not preferable as previously described.